A Phase I trial of endoxifen, an active metabolite of the cancer drug tamoxifen, indicates that the experimental drug is safe, with early evidence for anti-tumor activity. Mayo Clinic oncologist and lead author of the study Matthew Goetz, M.D., says, “We achieved up to sixtyfold higher levels of endoxifen compared to endoxifen levels achieved with the standard dose of tamoxifen. We have seen evidence for tumor regression in patients who had failed standard hormonal therapies including aromatase inhibitors, fulvestrant and tamoxifen. This is an exciting first step in the development of this drug.”